The present invention relates to an apparatus for monitoring and controlling a welding phenomenon in an electric resistance welding.
Previously, welding conditions in the electric resistance welding have been usually determined by operating personnel who conduct visual observation concerning spatter of a portion being welded or external flash of a weld bead and consider a size and a kind of material to be welded as well an anode voltage and an anode current which indicate a tendency of the welding phenomenon. To carry out these operations, considerable skillfulness and experience should be required as variations occur in respective operating procedures conducted by different operators, causing fatigue of the operator and therefore, it is difficult to carry out electric resistance welding under constant welding conditions even by skilled operators.
To improve these inferiorities appearing in the above-mentioned technique, a method was proposed, wherein a welding current or a temperature at a weld seam is measured and compared with a predetermined value corresponding thereto, and a control of a welding power source or a welding rate is carried out based on a deviation therebetween. However, there are several factors such as an upset amount, a Vee shape, manner of edge match, size and properties of material to be welded as well as the welding current or the welding rate, which complementingly interrelate with each other and therefore, important inferiorities as shown below appear when employing only the welding current or the weld seam temperature for determining the welding condition.
(i) The method for constantly controlling the welding current with respect to the predetermined value is not suitable in case there exists a variation in the upset amount due to a difference of the size or the properties of the material to be welded since such variation causes an optimal welding current to change.
(ii) The method for detecting the weld seam temperature is also unfavorable. The welding current must be increased in response to an increase in the welding rate. However, a temperature controller according to this method causes the welding current to decrease when increasing the welding rate because, even though the actual temperature does not change, the detected temperature becomes high since a cooling time corresponding to a traveling time of the material to be welded from a weld point to a temperature detecting point becomes short.
(iii) A complementary apparatus having many sensing elements and many calculating circuits is required in order to eliminate the above-mentioned disadvantages shown in (i) and (ii), when such improvement is conducted on the basis of only conventional technical thought which is as follows: To improve the abovementioned conventional techniques which do not respond to or do not correctly respond to the variation in the welding conditions, all of the variable factors relating to the welding conditions must be detected and calculations whose results are utilized for the control must be carried out by considering interrelationships existing among the variable factors since the feedback control wherein the welding current or the temperature is constantly maintained is not sufficient for conducting the precise control.
(iv) In addition, the above-mentioned control based on the detection of all variable factors in the welding is practically impossible since accurate calculating methods cannot be established without carrying out enormous trials and errors.
By the way, the following consideration relating to the welding phenomenon in the electric resistance welding has been previously accepted which is as follows: Strip edges to which the high frequency current is applied from V-shaped configuration and approach each other, being welded at a V-convergence point, while being subjected to efficient heating due to a skin effect and a proximity effect of the high frequency current.
In order to improve the conventional techniques, we have concentrated on the study of a basic welding phenomenon in the electric resistance welding by observing successive welding phenomenon consisting of heating, melting amd pressurizing at the strip edges in an area from a high frequency feeding point to a central portion of a squeeze roll with respect to a number of pipes having different sizes in diameter and thickness in different welding conditions which are the welding rate, the welding current, the position of the feeding point, the upset amount, and an amount of a Vee angle, by using a high speed photograph. As a result, we have found the welding phenomenon which is different from an ordinary conventional technical concept, on which we submitted a patent application which was laid open to public inspection as Japanese Patent Publication No. 111851/1977 and reported in conference held by the Iron and Steel Institute of Japan in October, 1977 (Lecture No. 232,233) and by the Japan Welding Society in November, 1977 (Lecture No. 408,409) in Japan.
Referring to the attached drawing, an explanation concerning the above-mentioned findings will be given. FIGS. 1a, 1b and 1c, are schematic views for illustration of the welding phenomenon, which respectively show a first-type welding phenomenon, a second-type welding phenomenon and a third-type welding phenomenon wherein P is a material or a pipe to be welded, V is a V-convergence point, W is a weld point, M and N are strip edges, X, Y are welding contacts, and S is a squeeze roll.
In each type of the welding phenomena mentioned above, there exists periodic movement of the weld point W with respect to the V-convergence point V which is formed by the strip edges M and N and whose position is constantly maintained.
When a change occurs in the welding phenomenon from the first type to the second and hence to the third type welding phenomenon, an amount of fluctuation period of the position of the weld point and a fluctuation width (a moving distance) become large.
By way of example, the fluctuation period corresponding to the phenomenon of FIGS. 1(a), 1(b) and 1(c) are, respectively, less than 1 msec, 1 msec-10 msec, and 10 msec-1000 msec.
Next, a detailed explanation as to the welding phenomenon will be shown. The strip edges electromagnetically repel each other since the high frequency current inversely flows in each strip edge and is high in current density, and the electromagnetic repulsive force on both of the edges causes molten metal to extrude toward an inner or outer surface of the strip, forming a weld bead after melting edge surfaces. Therefore, when increasing the amount of heat input and extruding some amount of the molten metal, a formation of a parallel slit gap directing from the V-convergence point to a central portion of the squeeze roll rather than welding of the slit gap at V-convergence occurs as shown in FIG. 1 (b). Under this situation, the weld point is produced at the tip portion of the slit gap which tends to move at a rate being the same as the welding rate, while molten metal for bridging across the parallel slit gap is frequently produced at the V-convergence point, said molten metal moves to the tip portion of the slit at a high moving rate due to the electromagnetic force. Thus periodic fluctuation of the weld point which is recognized to be included in the second-type welding phenomenon, occurs.
When increasing the heat input, the slit width becomes greater, the tip portion of the slit moves a considerable distance in association with the movement of the pipe but the strip edges are not welded during this time. When the distance between the V-convergence point and the weld point reaches a considerable amount, both of the strip edges suddenly contact at the V-convergence point where molten metal is formed bridging across the gap and said molten metal is rapidly moved to the weld point due to the electromagnetic force. Thus welding of the slit gap is effected in a very short time due to the movemet of the molten metal and due to a filling phenomenon of the slit gap with the weld bead when the electromagnetic force applied thereto disappears resulting from the bridging phenomenon. As a result, the tip portion of the slit is forced to return to near the V-convergence point.
The periodic repetition of the above-mentioned phenomenon produces the third-type of welding phenomenon. The periodic fluctuation in each welding phenomenon cannot be eliminated by constantly maintaining the welding conditions such as the heat input and the welding rate and by eliminating vibration due to eccentric arrangement of the rolls and the like since such periodic fluctuation is to be considered as one of the basic phenomena in the welding process.
We considered that fluctuation of the position of the weld point W is a primary factor which affects the shape of the weld bead, the spatter of the weld portion and the stability of the quality of the welded articles.
The positional fluctuation of the weld point W causes a periodic fluctuation in the shape of a welding current circuit formed in the material to be welded along a loop X-V-W-V-Y. In other words or from an electrical viewpoint, the positional fluctuation causes the periodical fluctuation in load impedance. Generally, the electric resistance welding machine utilizes a self-oscillation system and thus, in the oscillating frequency of high frequency used for the welding, and phase difference between the high frequency voltage and the high frequency current, periodical fluctuation is produced in association with the periodic fluctuation of the load impedance. From the relationship of (period)=1/(frequency), it is obvious that the high frequency oscillating period also contains the periodic fluctuation.
A measurement concerning at least one of the welding characteristics, which are referred to as "high frequency welding characteristics" hereinafter, consisting of the period, the frequency and the phase difference, permits a detection of the variation in the shape of the welding current circuit, thereby the type of welding phenomenon can be precisely grasped.
Concerning this measurement, we proposed in the above-mentioned Japanese Patent Application a method for detecting the variation in the frequency by using FM detection of either modulated high frequency current or modulated voltage and for detecting the variation in the phase difference between the current and the voltage by using a phase discrimination circuit.
After carrying out detailed researches and experiments with respect to the variation in the high frequency welding characteristics due to the above-mentioned welding phenomenon, we found that the amount of the variation in the high frequency welding characteristics is 0.01% to 0.1% in the second-type welding phenomenon and 0.1% to 0.5% in the third-type welding phenomenon.
The variation amount is substantially proportional to the distance between V and W shown in FIG. 1. And the shorter the period of repetitional fluctuation of the weld point W becomes, the lower the variation amount in the high frequency welding characteristics becomes.
Furthermore, we found that these relationships generally exist in the welding machine using the oscillating frequency above 100 KHZ regardless of the type of feeding system, such as a direct feeding or induction feeding, amount of supplied power, size and kind of pipe to be welded.
A detector available for measurement of these welding phenomena based on the above-mentioned facts must be provided with high accuracy to detect a very small variation in the order of 0.01% and frequency characteristics enough to detect the variation period of the weld point W in the order of 1 msec.
When utilizing output signals of the detector being proportional to the variation in the input signals (the high frequency welding characteristics) to control the welding machine, the occurrence of secular change or short time instability in parameters of structural components of the welding machine and change due to differences in pipe size must be eliminated.
It is very difficult to satisfy the above-mentioned requirements such as detection accuracy, frequency characteristics and stability of gain by using the FM detection system or an analog phase discriminator used in an audio device.